46499-G10
Spark Plasma Sintering of Ceria Materials for Solid Oxide Fuel Cells Applications
Solid oxide fuel cells (SOFCs) use protons or oxygen anions as charge carriers have the potential to become a power generation system of choice for the conversion of chemical to electrical energy. The challenge stays in the high temperatures (800-1000ºC) that SOFC require, which in turn lead to problems related to materials degradation. Several studies have been conducted in an attempt to reduce the operation temperature of SOFCs. Ceria (CeO2) has unique catalytic, electrochemical and storage characteristics that makes it attractive for a large array of applications, such as catalysts, solid oxide fuel cell systems, and gas sensors. Moreover, doping with rare earth elements leads to materials with superior chemical and physical stability, high oxygen mobility and high concentration of oxygen vacancies. For example, when doped with elements such as Gd, Yb or Sm, ceria displays ionic conductivity superior to zirconia, making it attractive as an alternative for medium-temperature applications in solid-state electrochemical devices. Nanostructured bulk cerium oxide has only recently been investigated by two research groups. Most of the studies on cerium oxide materials involved either mostly a very low dopant level or only considered one type of dopant. Here, we report on the spark plasma sintering (SPS) of bulk nanostructured ceria doped with three different rare-earth metals (M=Yb3+, Gd3+ and Sm3+) with doping levels of 20 wt%, and investigated the effect of the type of doping on their properties. The SPS experiments were performed at a maximum temperature of 1100ºC. The powders were synthesized by a modified sol-gel procedure starting with Ce(NO3)3×6H2O as a precursor. The SPS densification experiments were performed with a laboratory apparatus capable of delivering 4800 A and 150 kN load. In our system the load is delivered by a Universal test frame (Instron 5584, Instron Inc, USA) and the current is provided by programmable power supplies (Xantrex Inc, Canada).
Fully dense samples were obtained by SPS sintering at 1100ºC for all samples. SPS is a consolidation technique that uses a combination of pulsed electrical field application with high heating rates to densify, while preserving an ultra-fine grain size, a large variety of powder materials that were proved to be difficult to sinter by other methods. The technique uses a pulsed electric field application together with high heating rates and moderate pressures to sinter a large variety of powder materials, both conductive and non-conductive.
The SEM analysis showed that Sm-doped ceria has the largest grain size (~150 nm) after SPS sintering, despite the fact that the largest particle size in the initial powder was that of the gadolinium-doped sample. The initial powders were characterized by TEM and XRD and the sintered samples were characterized by XRD, SEM, and impedance spectroscopy.
The impedance spectroscopy results indicated that while ceramic materials usually display two semicircles in their impedance spectra (one for bulk and one for grain boundary effects), our results showed only one semicircle. The same effect was noticed by Anselmi-Tamburini et al. for SPS sintered Sm-doped CeO2. The total resistivity of the sample can be determined from the low-frequency intercept on the real axis in the impedance spectrum. We determined that that the samarium-doped ceria has the lowest total resistivity, and therefore the highest ionic conductivity. It has been suggested that the electrical field applied during SPS sintering has as a result a possible enhancement of grain growth in conductive samples. This effect is usually counterbalanced by the high heating rates used during this process. From our electron microscopy results, we found that Sm-doped ceria displays the largest percentage of grain growth upon SPS sintering. This is in good agreement with the AC impedance spectroscopy results that indicate the lowest total resistivity value in this sample. In summary, nanometric powders of ceria doped with 20% of three different doping elements (Sm, Gd, and Yb), gadolinium and ytterbium were synthesized by a solution method and fully sintered at 1100ºC by SPS. The samples were characterized to determine the influence of the type of doping element on the materials properties after SPS sintering. Despite the fact that Yb-doped sample had the largest initial particle size, the grain growth upon SPS sintering was promoted at a higher extent in the Sm doped sample, displaying the highest ionic conductivity. Yb doping produced the finest grain size in the SPS sintered samples, while displaying the lowest total ionic conductivity. More work is underway to investigate the influence of other SPS parameters and the effects of dopant type and doping level on the properties of ceria ceramics.
